1. Field of the Invention
The present invention relates generally to a lithographic projection apparatus and more particularly to a lithographic projection apparatus including two projection beams.
2. Description of the Related Art
Lithographic projection apparatus are used in the manufacture of integrated circuits (ICs), flat panel displays and other devices involving fine structures. The programmable patterning means generates a pattern corresponding to an individual layer of, for example, the IC, and this pattern is imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. silicon wafer or glass plate) that has been coated with a layer of radiation-sensitive material (resist).
Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake and measurement/inspection of the imaged features. These transfer procedures result in a patterned layer of resist on the substrate. One or more pattern steps, such as deposition, etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., follow, each of which is intended to finish, create or modify a layer of the device. If several layers are required, then the whole procedure, or a variant thereof, is repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.
A complete substrate may have of the order of 3×1013 pixels over its full area. To achieve an exposure throughput of about 5 wafers per hour using a programmable patterning means with about 30 to 40×106 pixels requires that the programmable patterning means be refreshed with new data approximately every 250 microseconds taking account of the time required for exposure and dead time, i.e. at a rate of 4 kHz. Given that each pixel requires a byte of data, the data must be transferred at a rate of about 4×1012 bits per second. To calculate this data from a file representing the mask pattern at the necessary rate is a difficult problem and requires a high speed, and therefore an expensive, computer.